Hot water storage tanks for use with solar collector systems have developed in two different types of direct heaters. A first type uses a preheater tank and also a conventional hot water tank. Heat from the solar collector is transferred to the water in the preheat tank. As the hot water from the conventional water heater is used, it is replaced with water from the preheater storage tank which in turn is replaced by cold water from the supply. In this configuration, heat from the collectors is used to preheat the water and store it in the preheater tank. As a result, the preheater tank has radiant heat losses. The water then goes to the conventional water tank where it is further heated electrically. This conventional water tank also has radiant heat loss. The combined heat loss from both the preheat tank and the conventional tank causes a low system efficiency.
In a second type of direct heater, water is circulated between the solar heater and the main or conventional hot water storage tank. Although the number of components are reduced, thereby reducing costs, and although the reduced number of components likewise reduces the radiant losses associated with component surface areas, the integrated system can have even lower efficiency. The problem lies in the temperature differential between the water in the collector panels and the water in the storage tank. When only a small differential exists, that is, the storage tank is maintained at a relatively high temperature compared with the temperature of the solar panels, the transfer of heat will be small and the efficiency of the system will be marginal. Since operating temperatures of solar panels can range in the area of 130.degree.-140.degree. F., it is even possible to have the solar collector cooling the hot storage if the supplemental heater is set to a temperature higher than that of the solar panels. In fact, the usual warming of the storage tank water greatly diminishes the storage capacity of a solar hot water system. As a result, the single storage tank method has proven to be inefficient to the point of being uneconomic.
The problem of single tank inefficiency has been attacked in various ways. An example is U.S. Pat. No. 4,253,446, Muller wherein a separating wall divides a single tank into an upper and lower portion. Although this arrangement improves the efficiency of the solar panel heat transfer, the problem of radiant heat loss remains. The overall functional effect of the Muller arrangement is to stack a preheater tank vertically below a main storage tank. Because of the loss of volume of the main storage, that is, the upper portion of the tank, it is necessary to increase the temperature of the upper portion (to maintain capacity) or to physically increase the volume of the upper portion. In either case, the upper portion will experience increased radiant heat loss and therefore decreased efficiency. What is needed is a tank providing a temperature gradient where cooler water is located near the storage tank surface and hot water is located away from the tank surface.